Paste this into a playground and let it run for 5 seconds. You’ll get a non-nil response *and* error.

URLSession.swift

import Foundation
import PlaygroundSupport

let bigFile = URL(string: "https://speed.hetzner.de/1GB.bin")!
let task = URLSession.shared.dataTask(with: bigFile) { (data, response, error) in
    print("data: \(data)")
    print("response: \(response)")
    print("error: \(error)")
}
task.resume()

DispatchQueue.main.asyncAfter(deadline: .now() + 5) { 
    print("Cancelling dataTask")
    task.cancel()
}
PlaygroundPage.current.needsIndefiniteExecution = true